I've discovered that for some login IDs when I view the databases and permissions for a domain account in Management Studio >> Security >> Logins, it looks like this account is not setup in any databases.

When I run sp_helplogins for that same ID I get a list of databases and roles.

I've run sp_change_users_login 'Auto_Fix' for the ID for a database listed in the sp_helplogins results and receive ...

The number of orphaned users fixed by updating users was 0.The number of orphaned users fixed by adding new logins and then updating users was 0.